Regexp是一個學(xué)習這種表達式的好工具。
Regexp是一個由100%純java正則式處理包，是Jonathan Locke捐給Apache軟件基金會的。 他最初開發(fā)這個軟件是在1996年， 它包括完整的Javadoc文檔，以及一個簡單的Applet來做可視化調(diào)試和兼容性測試.
2)RE類regexp包中非常重要的一個類，它是一個高效的、輕量級的正則式計算器/匹配器的類，RE是regular expression的縮寫。正則式是能夠進行復(fù)雜的字符串匹配的模板，而且當一個字符串能匹配某個模板時，你可 以抽取出那些匹配的部分，這在進行文本解析時非常有用。下面討論一下正則式的語法。
為了編譯一個正則式，你需要簡單地以模板為參數(shù)構(gòu)造一個RE匹配器對象來完成，然后就可調(diào)用任一個 RE.match方法來對一個字符串進行匹配檢查，如果匹配成功/失敗，則返回真/假值。例如：
RE r = new RE("a*b");
boolean matched = r.match("aaaab");
RE.getParen可以取回匹配的字符序列，或者匹配的字符序列的某一部分（如果模板中有相應(yīng)的括號的 話），以及它們的位置、長度等屬性。如：
RE r = new RE("(a*)b"); // Compile expression
boolean matched = r.match("xaaaab"); // Match against "xaaaab"
String wholeExpr = r.getParen(0); // wholeExpr will be 'aaaab'
String insideParens = r.getParen(1); // insideParens will be 'aaaa'
int startWholeExpr = r.getParenStart(0); // startWholeExpr will be index 1
int endWholeExpr = r.getParenEnd(0); // endWholeExpr will be index 6
int lenWholeExpr = r.getParenLength(0); // lenWholeExpr will be 5
int startInside = r.getParenStart(1); // startInside will be index 1
int endInside = r.getParenEnd(1); // endInside will be index 5
int lenInside = r.getParenLength(1); // lenInside will be 4
RE支持正則式的后向引用，如：
([0-9]+)=\1
匹配 n=n (象 0=0 or 2=2)這樣的字符串
3)RE支持的正則式的語法如下：
字符
unicodeChar Matches any identical unicode character
\ Used to quote a meta-character (like '*')
\\ Matches a single '\' character
\0nnn Matches a given octal character
\xhh Matches a given 8-bit hexadecimal character
\\uhhhh Matches a given 16-bit hexadecimal character
\t Matches an ASCII tab character
\n Matches an ASCII newline character
\r Matches an ASCII return character
\f Matches an ASCII form feed character
字符集
[abc] 簡單字符集
[a-zA-Z] 帶區(qū)間的 字符集
[^abc] 字符集的否定
標準POSIX 字符集
[:alnum:] Alphanumeric characters.
[:alpha:] Alphabetic characters.
[:blank:] Space and tab characters.
[:cntrl:] Control characters.
[:digit:] Numeric characters.
[:graph:] Characters that are printable and are also visible.(A space is printable, but not visible, while an `a' is both.)
[:lower:] Lower-case alphabetic characters.
[:print:] Printable characters (characters that are not control characters.)
[:punct:] Punctuation characters (characters that are not letter,digits, control characters, or space characters).
[:space:] Space characters (such as space, tab, and formfeed, to name a few).
[:upper:] Upper-case alphabetic characters.
[:xdigit:] Characters that are hexadecimal digits.
非標準的 POSIX樣式的字符集
[:javastart:] Start of a Java identifier
[:javapart:] Part of a Java identifier
預(yù)定義的字符集
. Matches any character other than newline
\w Matches a "word" character (alphanumeric plus "_")
\W Matches a non-word character
\s Matches a whitespace character
\S Matches a non-whitespace character
\d Matches a digit character
\D Matches a non-digit character
邊界匹配符
^ Matches only at the beginning of a line
$ Matches only at the end of a line
\b Matches only at a word boundary
\B Matches only at a non-word boundary
貪婪匹配限定符
A* Matches A 0 or more times (greedy)
A+ Matches A 1 or more times (greedy)
A? Matches A 1 or 0 times (greedy)
A{n} Matches A exactly n times (greedy)
A{n,} Matches A at least n times (greedy)
非貪婪匹配限定符
A*? Matches A 0 or more times (reluctant)
A+? Matches A 1 or more times (reluctant)
A?? Matches A 0 or 1 times (reluctant)
邏輯運算符
AB Matches A followed by B
A|B Matches either A or B
(A) Used for subexpression grouping
(?:A) Used for subexpression clustering (just like grouping but no backrefs)
后向引用符
\1 Backreference to 1st parenthesized subexpression
\2 Backreference to 2nd parenthesized subexpression
\3 Backreference to 3rd parenthesized subexpression
\4 Backreference to 4th parenthesized subexpression
\5 Backreference to 5th parenthesized subexpression
\6 Backreference to 6th parenthesized subexpression
\7 Backreference to 7th parenthesized subexpression
\8 Backreference to 8th parenthesized subexpression
\9 Backreference to 9th parenthesized subexpression
RE運行的程序先經(jīng)過RECompiler類的編譯. 由于效率的原因，RE匹配器沒有包括正則式的編譯類. 實際上， 如果要預(yù)編譯1個或多個正則式，可以通過命令行運行'recompile'類,如
java org.apache.regexp.recompile a*b
則產(chǎn)生類似下面的編譯輸出（最后一行不是）：
// Pre-compiled regular expression "a*b"
char[] re1Instructions =
{
0x007c, 0x0000, 0x001a, 0x007c, 0x0000, 0x000d, 0x0041,
0x0001, 0x0004, 0x0061, 0x007c, 0x0000, 0x0003, 0x0047,
0x0000, 0xfff6, 0x007c, 0x0000, 0x0003, 0x004e, 0x0000,
0x0003, 0x0041, 0x0001, 0x0004, 0x0062, 0x0045, 0x0000,
0x0000,
};
REProgram re1 = new REProgram(re1Instructions);
RE r = new RE(re1);
通過利用預(yù)編譯的req來構(gòu)建RE匹配器對象，可以避免運行時進行編譯的成本。 如果需要動態(tài)的構(gòu)造正 則式，則可以創(chuàng)建單獨一個RECompiler對象，并利用它來編譯每個正則式。注意，RE 和 RECompiler 都不是 threadsafe的（出于效率的原因）, 因此當多線程運行時，你需要為每個線程分別創(chuàng)建編譯器和匹配器。
3、例程
1)regexp包中帶有一個applet寫的小程序，運行如下:
java org.apache.regexp.REDemo
運行后：
2)Jeffer Hunter寫了一個例程，可以下載。
3)regexp自帶的測試例程，也很有參考價值。它把所有正則式及相關(guān)的字符串以及結(jié)果都放在一個單獨的文件 里，在$REGEXPHOME/docs/RETest.txt中。當然，這個例程的運行也要在$REGEXPHOME目錄下。
cd $REGEXPHOME
java org.apache.regexp.RETest